[DeTomaso] Fw: Stupid question

Thu Jun 26 08:15:31 EDT 2008

> Will
>
> The reason for the clearance is to insure that the valves have an 
> opportunity to fully seat.  Important for both cynlider sealing and, in 
> the case of the exhaust valve, transferring heat to the head in order to 
> prevent valve burning. If you tried to set the lash at zero, expansion of 
> materials may prevent the valves from seating.
>
> JT
> ----- Original Message ----- 
> From: "Will Kooiman" <wkooiman at earthlink.net>
> To: "'Pantera List'" <DeTomaso at realbig.com>
> Sent: Thursday, June 26, 2008 1:55 AM
> Subject: Re: [DeTomaso] Stupid question
>
>
>> Yeah, but the heads grow too.  Which grows more - the heads or the 
>> lifter,
>> pushrod, camshaft combination?
>>
>> I set my lash hot to spec, and then let the engine completely cool.  Once 
>> it
>> has completely cooled, I measure the lash.  From then on, I set the lash 
>> at
>> the cold setting.  It's a lot nicer setting the valves when the engine is
>> cold.
>>
>> My 351C runs about 0.009 less when cold.
>>
>> My 428 ran about 0.012 less when cold.
>>
>> Both of those have/had aluminum heads.
>>
>> I'm guessing cast iron would also grow more than the harder pushrods,
>> lifters, and camshaft, but probably not 0.009".
>>
>> At any rate, maybe it is for thermal expansion.  In my case, to prevent 
>> the
>> valves from sticking open if the engine gets too cold.  ;-)
>>
>> Oh yeah, we didn't talk about the other theory.  Wiping cam lobes. 
>> Doesn't
>> it make sense that solid cams would be less susceptible to wiped lobes?
>>
>> Will.
>>
>> -----Original Message-----
>> From: detomaso-bounces at realbig.com [mailto:detomaso-bounces at realbig.com] 
>> On
>> Behalf Of Asa Jay Laughton
>> Sent: Wednesday, June 25, 2008 11:26 PM
>> To: 'Pantera List'
>> Subject: Re: [DeTomaso] Stupid question
>>
>> Nice Questions Will, I'm sure Dan Jones will set us all straight.
>>
>> In a nutshell, thermal expansion.
>>
>> In a solid lifter arrangement, the cam grows, the lifter grows, the
>> pushrod grows, etc.  If you are set to zero lash, then if just one part
>> grows .001 your valve will now be open when it should be closed, causing
>> an intake leak or an exhaust leak, take your pick, either one is bad.
>>
>> In a hydraulic lifter arrangement, the lifter is designed to take a
>> certain amount of preload, because the center of the lifter is built to
>> move.  Thus when all the parts heat up, expand and grow, the pushrod
>> simply rides a little further down in the lifter without keeping the
>> valve open.
>>
>> Why do hydraulic lifter cam lobes wipe?
>> Because someone adjusted the rockers too tight, typically bottoming out
>> the lifter.  This is why it is so critical to properly prep hydraulic
>> lifter prior to installation, AND to pay very close attention to the
>> preload.  Here is the typical problem:
>>
>> 351C stock rockers are bolt-down for use on hydraulic cams.
>> You take the heads to a local shop to have a valve job.
>> They don't pay attention to valve stem installed height.
>> You bolt the rockers back down (not knowing some valve stems are now
>> taller than they should be).
>> You (inadvertently) have now taken all the collapsible distance out of
>> the lifter (you've bottomed it out cold, essentially making it a solid
>> lifter with zero lash).
>> You start the engine and it all runs fine, until the heat starts
>> expanding everything and then it starts wiping the lobe on the cam
>> because there is no clearance anymore.
>>
>> Why do some cams require different lash than others?  It depends on a
>> lot of factors, but generally if it's the same engine, lash should be
>> about the same.  Now if we start talking about radical difference in cam
>> profiles, some of the lash may be calculated for specific moments of
>> inertia to help prevent valve float.  (That momentum thing you were
>> dreaming of).  This is where my experience and knowledge base gets
>> fuzzy, so I'll let better experts chime in here.
>>
>> Hopefully this helped answer some of your questions.  Don't let them
>> keep you up late at night.
>> :)
>>
>> Asa Jay
>>
>> Asa Jay Laughton, MSgt, USAFR, Retired
>>
>> & Shelley Marie
>> Spokane, WA
>>
>> 1973 Pantera L 5533
>> [ASASCAT]
>>
>> ******************************
>> http://www.asajay.com
>> http://www.351c.info
>>
>>
>>
>>
>> Will Kooiman wrote:
>>> I have a stupid question that has been bugging me lately.
>>>
>>>
>>>
>>> Why does a solid lifter camshaft require clearance (lash)?  In other
>> words,
>>> why can't you install it at zero lash - or at least near zero?
>>>
>>>
>>>
>>> Of course, I know you can't, but why?  If anything is going to move 
>>> around
>>> at high rpm, it should create too much lash, not too little, right?  For
>>> example, you're not going to hit 7,000rpm and suddenly have your 
>>> pushrods
>>> grow 0.250.  They might flex a little, or you might have valve bounce, 
>>> but
>>> that would increase the lash, not decrease it.
>>>
>>>
>>>
>>> We all know, a hydraulic lifter camshaft has zero lash.  How can it get 
>>> by
>>> w/out lash, but a solid lifter camshaft can't?
>>>
>>>
>>>
>>> Is a solid lifter camshaft installed with lash, because it can be? 
>>> (sorta
>>> like the joke, why does a dawg lick. nevermind)
>>>
>>>
>>>
>>> The reason it has been bugging me is because I was thinking about wiped
>>> camshafts.  How many of these are hydraulic, and how many are solid? 
>>> All
>> of
>>> the incidents I can remember have been hydraulic.  Maybe solid lifter
>>> camshafts are less prone to wiping because of the lash.  It makes sense
>>> anyway.  Hydraulic camshafts get rubbed 360deg by the lifters.  Solids
>> only
>>> get rubbed about 140-150deg  (duration @ lash / 2)
>>>
>>>
>>>
>>> A bonus question is, why do different solid camshafts require different
>>> amounts of lash?  I'm guessing it's because that's what they were 
>>> designed
>>> to have.  Okay.  Why?  Why not grind everything for a lash of 0.020? 
>>> What
>>> advantage would there be to grinding a cam for 0.025?
>>>
>>>
>>>
>>> Another idea I had was to allow the lifter to build up some momentum to
>> act
>>> on the valve, but that's starting to sound silly.
>>>
>>>
>>>
>>> Like I said, they're stupid questions.  Now you know what I'm thinking
>> about
>>> on the way home from work.
>>>
>>>
>>>
>>> Will.
>>>
>>>
>>>
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